The only version that is "made" to run on 2k/xp is loopy landscapes with the newest patch..anything else wont work (well...thast not ENTIRELY true..but we wont get into that)...
go to their website and download & install the patch...
I could run RCT on XP w/ no patch (but i also just bought it, so it's the newest "store" available version) but installed it w/ the patch to make it run on 2k..
I couldn't find anything on Now Inc.'s site but on the official RCT site go to Tech Support then patches and utilities and download the latest patch you can find, I did this and now it works fine on XP.